It is well established that surfaces can support mechanical waves. Depending on the nature of the substrate, these surface waves are called Rayleigh waves, Lamb waves, and Love waves. Rayleigh waves are for thicker substrates while Lamb and Love waves are for thin membranes. The surface waves can be generated on a piezoelectric substrate using electrode arrays deposited on the surface. Once created, the wave travels on the surface and can be detected using a second electrode array. If the surface waves encounter adsorbed species, the frequency of the wave varies very sensitively due to mass or stress loading. The frequency of the wave can be very high—hundreds of MHz—yet the frequency and phase can be measured very precisely to obtain extremely high sensitivity.
One important disadvantage of the above surface-wave sensor is its inability to distinguish between different chemical species. This disadvantage also exists on other mass sensors such as the quartz crystal microbalance (QCM), plate wave resonator, and Lamb and Love wave sensors. To overcome this disadvantage of lack of chemical selectivity, SAW sensors are generally coated with a chemically selective layer. However, this chemically selective layer frequently does not provide good chemical selectivity except in rare cases, for example, biosensors based on antibody-antigen interaction or DNA hybridization. Polymeric coatings are used in certain vapor sensors and are sensitive to a variety of analytes depending upon the solubility of the analyte in each polymer. To provide unambiguous detection of even a single analyte in a mixture that may contain other analyte vapors, for example, arrays of sensors with various polymers are required to provide even qualitative information about the nature of the adsorbates and thus the vapors.
Surface waves can also be extremely sensitive to temperature, depending upon the nature of the piezoelectric substrate. With appropriate material, small changes in temperature can cause rather large changes in frequency, wave velocity, and phase. In this invention, we exploit this extremely high sensitivity of surface waves to temperature and other physical properties to achieve chemical selectivity using photo-absorption. In addition to temperature, the surface wave velocity is affected by pressure, surface stress, magnetic and electric fields. This invention exploits changes in surface wave velocity due to variation in surface temperature, pressure, surface stress, electric and magnetic field induced by interaction of adsorbate with impinging electromagnetic radiation on the sensor surface. A variation in surface acoustic wave properties such as frequency, velocity, phase, amplitude, and Q-factor as a function of the wavelength of the impinging electromagnetic wave is used as a signatures for speciation of adsorbed species.
U.S. Pat. No. 6,180,415 to Schultz et al.; U.S. Pat. No. 6,331,276 to Takei et al.; and U.S. Pat. No. 6,515,749 to Pipino teach sensor devices and methods using only optical parameter changes for detection. U.S. Pat. No. 5,923,421 to Rajic et al., herein incorporate by reference teaches similar absorption spectrometry devices and methods using bolometers, thermopiles, pyroelectrics or microcantilever sensing elements.